Traditionally, low cost speed control of single phase induction motors has been accomplished by motor voltage reduction utilizing a triac. More specifically, the motor slows down as the terminal voltage to the motor is reduced. This technique works well for loads, such as fans and centrifugal pumps, where the load torque required drops smoothly and rapidly as speed is reduced.
One prior art triac control reduces the motor voltage by delaying conduction until after the line voltage crosses zero. At some point after the zero crossing, the triac is switched on (as determined by the control), and it remains on until the current in the triac goes to zero. This process is repeated every ½ cycle of the line voltage.